Ref parameter incorrectly decorated with `noalias` attribute

[RalfJung]

The following library:

use std::cell::*;

pub fn break_it(rc: &RefCell<i32>, r: Ref<'_, i32>) {
    // `r` has a shared reference, it is passed in as argument and hence
    // a barrier is added that marks this memory as read-only for the entire
    // duration of this function.
    drop(r);
    // *oops* here we can mutate that memory.
    *rc.borrow_mut() = 2;
}

generates IR for break_it that starts like

; playground::break_it
; Function Attrs: noinline nonlazybind uwtable
define void @_ZN10playground8break_it17hb8a9beeb2affda19E(
  { i64, i32 }* align 8 dereferenceable(16) %rc,
  i32* noalias nocapture readonly align 4 dereferenceable(4) %r.0,
  i64* nocapture align 8 dereferenceable(8) %r.1)
unnamed_addr #1 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {

Note the noalias. That is incorrect, this function violates the noalias assumptions by mutating the data r points to through a pointer (rc) not derived from c. Here's a caller causing bad aliasing:

pub fn main() {
    let rc = RefCell::new(0);
    break_it(&rc, rc.borrow())
}

RefMut has the same problem when -Zmutable-noalias is set.

Cc @rkruppe @comex

[gnzlbg]

cc @eddyb

[bjorn3]

Maybe pour an UnsafeCell into Ref instead of &T?

[RalfJung]

It should probably use a raw pointer (something I have been arguing for for years now ;). For RefMut, UnsafeCell wouldn't even help.

[gnzlbg]

Changing Ref value field to value: &'b std::cell::UnsafeCell<T> fixes this.

[RalfJung]

Also see #60076: vec_deque::Drain has a similar problem.

Changing Ref value field to value: &'b std::cell::UnsafeCell fixes this.

It does, but I think a raw pointer is the better fix.

[gnzlbg]

It does, but I think a raw pointer is the better fix.

Why?

The value field inside RefCell is of type UnsafeCell<T>, not T, so IMO Ref has a bug by using &T instead of &UnsafeCell<T>. Using a raw pointer (e.g. NonNull<T>) would mean that we lose the dereferenceable annotation.